The human teratocarcinoma cell line PA-1 was found to contain a dominant trans forming gene when analyzed in the NIH3T3 tissue culture DNA transformation assay. Isolation of the transforming DNA sequences by molecular cloning is in progress. Two of the four RI fragments present in a secondary focus NIH3T3 cell have been cloned. These will be used to isolate the complete transforming region from a genomic library prepared from this cell line.